The use of infrared (IR) filters for cameras is well known in the art, and in particular in relation to digital cameras. In short, the image sensor of the camera has a spectral response with a non-negligible component in the infrared. This results in opportunities as well as challenges. An opportunity lies in that in low-light conditions the IR-component may provide useful information about the imaged scene, information which may be enhanced even further by means of an IR illuminator. A challenge is found during day-time imaging, where the addition of an IR-component will distort the color balance in the image, and it may also saturate the image sensor completely.
A way of maintaining the beneficial effects while suppressing the less beneficial effects is to add a movable IR-cut filter in the beam path in front of the image sensor. In this way, the IR (cut) filter may be used during daylight conditions, enabling acquisition of color images. Through-out the application “IR-cut filter” and “IR-filter” may be used interchangeably and the context will render the meaning obvious. With an IR-cut filter arranged, pixels of the image sensor may operate in a first manner, in which the incident light is divided into colors and detected as a charge on individual photodetectors thus enabling color separation. During night time, or in low-light conditions, the IR filter may be removed and use is made of the resulting increase in incoming radiation from the IR portion of the spectrum. The IR radiation will not contain any color information and it will distort the color presentation, so instead the intensity information will be presented as a grayscale intensity image (or with any desirable color scale). This also means that the color channels that were used to generate the color for one pixel are combined into a single intensity signal. To further improve the night mode performance an IR light illuminator may be added.
In the straightforward solution, an IR-cut filter is arranged in front of the sensor, and an actuator is used for moving the filter between a position in which it completely covers the sensor (‘day mode’) and a position in which it is completely removed from obscuring the image sensor (‘night mode’). This on-or-off approach is believed to be the most common approach in commercially available products, though the approach does have some accompanying artifacts. When switching from night mode to day mode it is not uncommon that the light level with the IR-filter in the beam path is too low, and that a switch back to night mode is necessary, resulting in a back-and-forth flickering between night mode and day mode. The distinct “click” sound associated with the switch may add to the sometimes annoying effect of temporally poor image quality. There is also considerable wear on the actuator. A current solution would be to add a light sensor, so that it may be deduced that the light level is acceptable prior to effecting the switch. In patent literature, there are examples of more elaborate examples.
In JP patent publication No. 2006078666, an arrangement where the IR-filter is arranged adjacent to a diaphragm (an iris diaphragm) is disclosed. The IR-filter has a radially non-uniform transmission profile where the center of the filter has essentially no transmission while the transmission increases with the radius (not necessarily in a linear fashion). The IR-filter is arranged concentrically with an optical axis, which is also the case for the diaphragm. The consequence is that when there is an abundance of ambient light and the diaphragm has a very small opening, the IR-component will be removed from all radiation passing the diaphragm and reaching the image sensor. As the amount of ambient light reduces, it will be possible to switch to night mode by simply increasing the size of the diaphragm, reducing the IR-attenuating portion to a minor portion of the diaphragm opening.
There is an added benefit in having the filter close to the plane of the diaphragm (referred to as aperture plane or aperture stop in the following), since in a normal configuration the aperture plane represents a position where there is no spatial correlation with the plane of the image sensor. Sometimes this is referred to as the aperture stop, wherein the aperture stop limits how much light from each point of the object that reaches a conjugate image point (the sensor in our case). Aperture stop thus defines a plane in the beam path, and sometimes the term “aperture plane” is used for the same feature. A feature of this plane is that it is a position in which alterations in a size of a diaphragm aperture will affect the entire image plane equally, at least in an ideal situation.
The disclosed teachings mainly relate to an improved IR-filter arrangement.